1. Field of the Invention
The present invention relates to an encoder such as a rotary encoder, a linear scale, and the like, and, more particularly, to an encoder that can output the information of noise level.
2. Description of the Related Art
Encoders are well known in the prior art as devices for detecting the moving position or the amount of displacement of a moving body, and can be roughly classified into two types: rotary encoders are encoders that detect the angle of rotation or the amount of rotation; and linear scales are encoders that detect the position or the amount of displacement from a reference position along a path.
Generally, an encoder is mounted on the housing of a driving device such as a motor that drives a moving body.
An encoder contains a circuit board, and a signal processing circuit is mounted on this circuit board. Usually, there is no direct electrical connection between the circuit board contained in the encoder and the housing on which the encoder is mounted, but they are electrically connected indirectly via a stray capacitance.
In this case, if a potential variation due to noise, etc. occurs on the housing side, the encoder circuit may be affected via the stray capacitance, to cause an error in the position data or displacement amount data that the encoder outputs.
FIG. 11 is a schematic diagram for explaining a prior art encoder. In FIG. 11, the encoder 101 comprises a movement detecting means 102 for detecting the movement of a moving body 10 and a signal processing circuit 103 for generating an encoder signal by processing the movement detection signal supplied from the movement detecting means 102; here, the signal processing circuit 103 is mounted on a circuit board 106.
The movement of the moving body 10 driven by a driving device 11 such as a motor is detected by the movement detecting means 102, and the encoder signal is generated by the signal processing circuit 103.
FIG. 12 is a schematic diagram for explaining how the prior art encoder is mounted on a housing. The encoder is mounted on the housing 13 of the driving device (hereinafter referred to as the motor) via an encoder flange 103a provided on the encoder. The encoder flange 103a is fixed to the motor housing 13 with flange fixing screws 103b, and the circuit board 106 is fixed to the encoder flange 103a with circuit board fixing screws 103d. Here, a rotating slit 103c is mounted between the circuit board 106 and the encoder flange 103a. 
FIG. 13 shows one example of noise occurring in the motor housing. In the prior art, as shown in FIG. 12, for example, the noise is measured by directly measuring a noise potential with a potential measuring means 104 connected between the motor housing 13 and a 0-volt point on the circuit. The measured noise potential is used to take remedial measures against the noise based on the level of the noise potential.
Further, as a technique for detecting noise contained in the encoder signal, it is known to detect noise introduced in an encoder line along which the encoder signal is sent to the control unit (refer to Japanese Unexamined Patent Publication No. 2000-258481).
There is no direct electrical connection between the signal processing circuit in the encoder and the housing on which the encoder is mounted, but they are electrically connected indirectly via a stray capacitance.
Accordingly, if the motor is not properly grounded, noise (potential difference) occurs in the housing. This noise may affect the encoder circuit via the stray capacitance, causing an error in the position data or displacement amount data that the encoder produces.
In the prior art, the potential of the motor housing is directly measured using a measuring device, and remedial measures are taken to reduce the noise by making a judgment based on the noise level obtained through the measurement.
The technique for measuring the noise introduced in the encoder line connecting between the encoder and the control unit is known in the above-cited Japanese Unexamined Patent Publication No. 2000-258481, but the technique disclosed in the above-cited document is one that addresses communication system-related noise, and therefore, cannot measure the noise occurring within the encoder.
As the noise occurring within the encoder is included in the encoder signal itself, it is not possible to discriminate the noise from the signal representing the position data or displacement amount data of the moving body and, once output on the encoder line, the noise cannot be measured with the technique disclosed in Japanese Unexamined Patent Publication No. 2000-258481.